Prosthetic heart valves are typically used to replace diseased natural heart valves in either the aortic or mitral position. Several categories of prosthetic heart valves are in existence. One category includes what may be referred to as mechanical heart valves. Such valves typically have a rigid orifice ring and rigid hinged leaflets coated with a blood compatible substance such as pyrolytic carbon. Other configurations, such as ball-and-cage assemblies, have also been used for such mechanical valves.
A second category of prosthetic heart valves comprises assemblies having various amounts of biological or natural material. As described in more detail below, some of these valves include leaflets derived from natural material (typically porcine) and still include the natural supporting structure or ring of the aortic wall. In other valves, leaflets derived from natural material (typically bovine pericardium) are trimmed and attached to a synthetic, roughly annular structure or ring that mimics the function of the natural aortic wall. In still other valves, both the leaflets and the annular support ring are formed of synthetic polymers or biopolymers (e.g., collagen and/or elastin). For ease of description, these valves will be referred to herein as bioprosthetic valves.
Many bioprosthetic valves include an additional support structure or stent for supporting the leaflets, although so-called stentless valves are also used. The stent provides structural support to the cross-linked valve, and provides a suitable structure for attachment of a sewing cuff to anchor or suture the valve in place in the patient.
Bioprosthetic valves which include a stent are typically of two types. In one type, an actual heart valve is retrieved from either a deceased human ("homograft") or from a slaughtered pig or other mammal ("xenograft"). In either case, the retrieved valve may be trimmed to remove the aortic root, or the aortic root or similar supporting structure may be retained. The valve is then preserved and/or sterilized. For example, homografts are typically cryopreserved and xenografts are typically cross-linked, typically in a glutaraldehyde solution. The tissue valve may then be attached to the stent.
The other type of stented bioprosthetic valve includes individual valve leaflets which are cut from biological material, e.g., bovine pericardium. The individual leaflets are then positioned on the stent in an assembly that approximates the shape and function of an actual valve.
In the case of either type of stented bioprosthetic valve, the function of the stent is similar. Primarily, the function of the stent is to provide a support structure for the prosthetic valve. Such a support structure may be required because the surrounding aortic or mitral tissue has been removed in harvesting the valve. The support offered by a stent in a valve is important for several reasons. First of all, a valve is subject to significant hemodynamic pressure during normal operation of the heart. Upon closing the valve the leaflets close to prevent backflow of blood through the valve, In the absence of any support structure, the valve cannot function properly and will be incompetent. One function of the stent is to assist in absorbing the stresses imposed upon the leaflets by this hemodynamic pressure. This is typically achieved in existing stents through the use of commissure support posts to which the valve commissures are attached.
Some known stents have been designed such that the commissure support posts absorb substantially all the stresses placed on the valve by hemodynamic pressure. One such stent is a formed piece of spring wire which is bent to form three vertically-extending commissure support posts, each having a U-shape and being connected to the other commissure support posts via arcuate segments of wire. Such a stent is described in U.S. Pat. No. 4,106,129 to Carpentier, et al. In that stent, the leaflet stresses are borne by the commissure posts rotating around and exerting a torque upon the arcuate wire sections between the posts. The composition and structure of this stent also provides for deformability of the orifice-defining elements. A separate insert element in the form of a plastic web is positioned around the wire stent prior to attachment of the valve.
In other types of stents, the commissure posts are fixed to a rigid base and are designed to be substantially flexible along their entire length so that the posts bend in the manner of a fishing pole in response to the stresses imposed upon the leaflets by hemodynamic pressure. An example of such a stent is shown in U.S. Pat. No. 4,343,048 to Ross, et al.
Other stents, for example the stent shown in U.S. Pat. No. 4,626,255 to Reichart, et al., include further support structure connected to and disposed between the commissure support posts. Such support structure prevents a given commissure post from being resilient along its entire length. Still other stents, such as in U.S. Pat. No. 5,037,434 to Lane, include an inner support frame with commissure posts resilient over their entire length, and a relatively more rigid outer stent support which begins to absorb greater stress as the associated commissure support bends further inward.
Although all of these stents provide support to the bioprosthetic valves to which they are attached, the stress distributions are often unnatural, leading to premature wear or degradation of over-stressed portions of the valve. Accordingly, the need exists for stents which more closely approximate the stress response of a natural aortic or mitral valve. Furthermore, the stents which include several parts are mechanically complex and require multiple assembly steps. A stent which includes a stress response that approximates a natural valve would thus also desirably have an integral construction.
Another function of a stent is to serve as a framework both for attachment of the valve, and for suturing of the valve into place in the recipient, e.g., a human patient. Toward that end, the stent, or a portion of the stent, is typically covered with a sewable fabric or membrane, and may have an annular sewing ring attached to it. This annular sewing ring serves as an anchor for the sutures used to attach the valve to the patient.
A variety of different stent designs have been employed in an effort to render valve attachment, and implantation of the valve simpler and more efficient. Design trade-offs have often occurred in designing these stents to have the desirable stress and strain characteristics while at the same time having a structure which facilitates assembly and implantation.
In the stentless valves previously referred to, the unsupported valve is sewn into the recipient's aorta in such a way that the aorta itself helps to absorb the stresses typically absorbed by a stent. Current porcine aortic stentless valves, such as porcine aortic stentless valves, are typically intended for use in the aortic position and not in the mitral position. A mitral valve would require a support structure not presently available with porcine aortic valves, and recently, stentless porcine mitral valves for placement in the mitral position have been developed.
Indeed, the stented valves used in the mitral position utilize the stent to provide support for normal valve function. In these stented mitral valves, a "low profile" stent having generally shorter commissure posts has been used, so as to prevent the ventricle wall from impinging on the valve. However, use of a lower profile stent often requires that the bioprosthetic valve be somewhat distorted upon attachment to the low-profile stent. This, in turn, can lead to reduced functionality of such valves. While the "higher profile" stents can avoid this distortion, care must be given to valve placement so as to avoid the referenced impingement by the ventricle wall. A need exists for a stent for use in the mitral position that includes the advantageous stress/strain and attachment characteristics previously described.
Known stents for bioprosthetic valves have been formed from a variety of materials including both metals and polymers. Regardless of the material employed, the long-term fatigue characteristics of the material are of critical importance. Unusually short or uneven wear of a stent material may necessitate early and undesirable replacement of the valve. Other material characteristics are also considered in selecting a stent material, including: rate of water absorption, creep, and resilience to the radiation which may be used for sterilization. Further, it may be highly desirable to form the stent of a radio-opaque material to allow the stented valve to be viewed by x-ray imaging. Of course, the selected material must also be biocompatible and have the required physical characteristics to provide the desired stress/strain characteristics. Furthermore, most existing stents are formed of a material having a constant cross-sectional dimension. Formed wire stents and stents formed from stamped metal are examples. Use of a material of variable cross section would allow stress and strain characteristics to be carefully controlled by adding or subtracting cross sectional area in certain regions of the stent, as may be required.